While Scotland Yard officers did talk to the man, they deemed his action to be "within the law".

When asked whether people should be free to carry around the ISIS flag, Mr Johnson told LBC Radio: "I don't like people carrying the ISIS flag... but it's not for me to make operational decisions on behalf of the police.

"I've made my views plain, I don't want people waving the ISIS flag."

But then when pressed on why we should not ban the ISIS flag, Mr Johnson said: "We live in a free country. You would have to have primary legislation to designate certain bits of iconography as being illegal."

He continued: "In the [London] Pride march you will recall the police did in fact confiscate a flag in the belief that it was an Isis flag and it turned out to be a display of various sex aids.

"So you have got to be careful how you approach this."

Mr Johnson's view appears to contradict David Cameron, who last year insisted that anyone caught flying an ISIS flag would be arrested.

I've made my views plain, I don't want people waving the ISIS flag

Boris Johnson

The Mayor of London also used his weekly interview show with Nick Ferrari to discuss the London Underground strike that is set to cause chaos for thousands of commuters.

Mr Johnson suggested the walk-out was nothing more than "payback" from trade unions who were furious at the Conservative's victory at the General Election.

Around 20,000 workers from several transport unions are due to walk-out from 6:30pm today in what could be the most troublesome strike in more than a decade.

The strike, which is expected to last 24 hours, stems from an ongoing dispute over the all-night Tube services, which are due to begin in September.

The Rail, Maritime and Transport union (RMT), Aslef, Transport Salaried Staffs Association (TSSA) and Unite, are currently at loggerheads with Tube chiefs over the new working timetable and the pay being offered for the service.

Mr Johnson said: "They can strike until they are blue in the face. The Night Tube will go ahead."